Saudi Arabia has been leading a coalition of Arab states since last Thursday in an air campaign against the Shi'ite Houthis, who emerged as the most powerful force in the Arabian Peninsula's poorest country when they seized Yemen's capital last year.

The Saudis say their aim is to restore President Abd-Rabbu Mansour Hadi, who left the country last week. The Houthis are allied with Saudi Arabia's regional foe Iran, and backed by army units loyal to longtime ruler Ali Abdullah Saleh, who was pushed out three years ago after "Arab Spring" demonstrations.

The conflict has brought civil war to a country already on the verge of chaos and forced Washington to evacuate its personnel from one of the main battlefields in the covert U.S. drone war against al Qaeda.

Residents and tribal sources in north Yemen reported artillery and rocket exchanges along several stretches of the Saudi border. Explosions and heavy gunfire were heard and Saudi helicopters flew overhead, they said.

In the southern port of Aden, Houthi fighters and allied army units pressed an offensive against forces loyal to Hadi, trying to capture the last remaining major stronghold of the absent president's forces.

At least 36 people were killed when Houthi forces shelled Hadi loyalists in Aden. Jets from the Saudi-led coalition bombed Houthi positions near the airport.

Further west, Houthi fighters entered a coastal military base overlooking the Red Sea's strategic Bab el-Mandeb strait, local officials said, when soldiers of the 17th Armoured Division opened the gates to the facility.

ENERGY GATEWAY

The Bab el-Mandeb shipping lane, which connects the Red Sea with the Gulf of Aden and Arabian Sea, is a vital energy gateway for more than 3 million barrels of oil passing daily to Europe, Asia and the United States.

Hadi's rump government, now based in Saudi Arabia, called for Riyadh to escalate the air war into an invasion.

Asked by an interviewer on pan-Arab television channel al-Arabiya Hadath whether he sought an Arab ground intervention, Yemeni Foreign Minister Riyadh Yaseen responded: "Yes, we are asking for that, and as soon as possible, in order to save our infrastructure and save Yemenis under siege in many cities."

Saudi authorities say they have gathered troops along the border in preparation for any possible ground offensive, but have given no timetable to send them in. Pakistan has also said it is sending troops to support Saudi Arabia.

"There could be a limited ground operation, in specific areas, at specific times. But don't expect there to be an automatic resort to a ground operation," said Brigadier General Ahmed Asseri, spokesman for the Saudi-led coalition.

"I don't want us to concentrate on the land operation as if it is a 'must' ... if it is possible to achieve the goals through other means."

In the southern city of Dhalea, residents reported heavy fighting, with southern secessionist fighters trading artillery fire with Houthis backed up by army units loyal to Saleh.

Repeated air strikes hit Houthi and allied positions, including an ammunition store at a military base, which caused huge explosions. An eyewitness said nine southern fighters were killed, along with around 30 Houthi and allied fighters.

In the east of the country, on the border between Shabwa and Marib province, at least 15 Houthi gunmen and their allies were killed in a clash with tribal fighters, local sources said.

The Houthis are backed by military units still loyal to Saleh, himself a member of their Zaidi sect, who fought to crush the Houthis while in power but has now allied with them.

YEARS OF UNREST

Saudi Arabia has a history of wielding influence in its poorer neighbor and fought a brief and indecisive ground conflict against the Houthis in the border area in 2009 while supporting then-leader Saleh.

The civil war comes after years of unrest and disintegrating central authority in a country also dealing with tribal discontent and al Qaeda's most potent regional branch, as well as a southern secessionist movement.

Saleh's decision to ally with the Houthis tips the regional balance of power away from Saudi Arabia and toward Iran, a feud also being played out on battlefields in Syria and Iraq. The crisis is the first big foreign policy test for Saudi Arabia's new king, Salman, and the kin he has elevated to top posts.

Iran's Deputy Foreign Minister Hossein Amir Abdollahian called the Saudi strikes a "strategic mistake". He said Tehran had a proposal to end the conflict and was trying to reach out to Riyadh. He gave no details.

"Iran and Saudi Arabia can cooperate to solve the Yemeni crisis," he said in Kuwait. "We recommend all parties in Yemen return to calm and dialogue."

Saudi Arabia's foreign minister, Prince Saud al-Faisal, said the operation would continue until it restored security and unity to Yemen.

"We are not the ones calling for war. But if you bang the drums of war, we are ready for it," he told the kingdom's Shura Council advisory body.

While the strikes have not halted the Houthi advance, the Saudi-led coalition says it has succeeded in closing off Yemeni airspace to Houthi supporters and imposing a naval blockade.

The International Committee of the Red Cross said one of its planes had been prevented from delivering medical supplies in Sanaa, and called for "the urgent removal of obstacles to the delivery to Yemen of vital medical supplies needed to treat casualties".

It also called on all combatants to allow humanitarian workers to operate safely. A Yemeni Red Crescent volunteer was shot dead on Monday in Dhalea while evacuating wounded people.